It is known practice to identify the orientation of the geographic North with a North seeker comprising a gyrometer connected to calculating means, or else with a stellar observation system.
The identification of the North with a gyrometer seeker relies on a process of measurement of the component of the terrestrial rotation vector in a horizontal plane, i.e. tangent, to the Earth at the site where the device is installed, this component being directed toward the geographic North.
Such means may be used with infantry binoculars: the North seeker and the binoculars rigidly attached together and connected together are held up by a tripod, which makes it possible to determine the bearing of a target. When this target is seen in the binoculars, the knowledge of the orientation of the geographic North, i.e. the knowledge of the orientation of the binoculars with respect to the geographic North, corresponds to the bearing where the target is found.
In this context, the angular accuracy required is of the order of milliradians. Possible spurious movements, i.e. the movements of the gyrometer with respect to the ground, such as for example the sinking of the tripod into the ground, are likely to falsify the measurement carried out by the gyrometer in the process of initialization of the orientation value of the binoculars with respect to the geographic North.
To avoid these spurious movements being interpreted by the gyrometer as being due to the rotation of the Earth, a precision accelerometer is also provided. This accelerometer makes it possible to identify the spurious movements so as to distinguish them, so that they do not falsify the measurement of the Earth's rotation performed by the gyrometer.
Moreover, a limitation of known North seekers lies in the fact that the phase of initialization of the value of orientation of the North is carried out. on the spot, so that if the binoculars are moved after this initialization, the information about the orientation of the North that they use is erroneous.